The present invention relates generally to circuitry for controlling automotive ignition systems, and more specifically to circuitry for compensating for undesirable high temperature operating effects associated with such systems.
Modern inductive-type automotive ignition systems typically control the ignition coil such that coil current is allowed to increase to a level high enough to guarantee sufficient spark energy for properly igniting an air/fuel mixture. The inductive nature of an ignition coil dictates that the coil current will increase over time, wherein a control circuit is typically operable to terminate coil charging after a so-called xe2x80x9cdwell timexe2x80x9d and thereby initiate a spark event, or to dynamically maintain the coil current at a predefined current level for a predefined time period before initiating a spark event.
In either case, ignition control circuits typically include a protection feature operable to prevent damage to the ignition controller circuitry or to the ignition coil itself in the event of a fault that could cause the coil to remain in a conductive state for prolonged periods of time. Such a protection feature is commonly implemented by a simple timing function that shuts off the drive signal to the coil current switching device after a predetermined time period has elapsed since activation thereof.
This xe2x80x9cover-dwellxe2x80x9d protection time must be guaranteed to be longer than the longest expected dwell period required by the ignition system for proper charging of the ignition coil. If the over-dwell protection period is too short, there may be insufficient energy in the ignition coil to ignite the air-fuel mixture, or the engine spark timing may be compromised in a fashion that creates emission problems. On the other hand, if the over-dwell protection period is too long, the ignition coil and/or controlling electronics may over heat and consequently become damaged. In either case, the protection circuitry has failed at its primary purpose.
Due to the relatively long over-dwell protection times required for engines operating in very low RPM or xe2x80x9ccrankxe2x80x9d modes; e.g., several tens of milliseconds, the over-dwell protection circuit may require a capacitor external to the integrated ignition control circuit. One known example of an ignition system 10 of the type just described is illustrated in FIG. 1, wherein system 10 includes an ignition control circuit 14 receiving an electronic spark timing (ES) signal from a control circuit 12 such as a microprocessor or microprocessor-based control circuit. The ignition control circuit 12 is responsive to the EST signal to supply a gate drive signal GD to a gate 16 of at least one insulated gate bipolar (IGBT) transistor 18 or other coil switching device. A collector 20 of IGBT 18 is connected to one end of a primary coil 32 forming part of an automotive ignition coil 30 having an opposite end connected to battery voltage VBATT. The primary
coil 30 is coupled to a secondary coil 34 having opposite terminals connected to opposing electrodes of an ignition plug 36 defining a spark gap therebetween. An emitter 22 of IGBT 18 is connected to one end of a sense resistor Rs having an opposite end connected to ground potential, and to circuit 14. System 10 may include additional IGBT and ignition coil pairs, as is known in the art, and circuit 14 is also connected to an external capacitor CEXT referenced at ground potential.
In the operation of system 10, the ignition control circuit 14 is responsive to a rising edge of an EST signal to supply a full gate drive signal GD to the gate 16 of IGBT 18. As IGBT 16 begins to conduct in response to the gate drive signal GD, a coil current Ic begins to flow through primary coil 32, through IGBT 18 and through Rs to ground, thereby establishing a xe2x80x9csense voltagexe2x80x9d Vs across resistor Rs. As the coil current Ic increases due to the inductive nature of coil primary 32, the sense voltage Vs across Rs likewise increases until it reaches an internal voltage VREF. At this point, the ignition control circuit 14 causes the gate drive circuit 20 to turn off or deactivate the gate drive voltage GD so as to inhibit the flow of coil current Ic through the primary coil 32 and coil current switching device 18. This interruption in the flow of coil current Ic through primary coil 32 causes primary coil 32 to induce a current in the secondary coil 34, wherein the secondary coil 34 is responsive to this induced current to generate an arc across the electrodes of the ignition plug 36. The ignition control circuit 14 further includes over-dwell protection circuitry operable to selectively charge and discharge capacitor CEXT at a rate defined by the EST signal. If EST remains in an active state for an excessive, or over-dwell time period, the charge on CEXT reaches a level that causes the ignition control circuit to gradually deactivate IGBT 18 to thereby gradually decrease the coil current Ic so as not to generate a spark event. Further details relating to the structure and operation of one known ignition control circuit of the foregoing type are provided in U.S. Pat. No. 5,819,713 to Kesler, which is assigned to the assignee of the present invention, and the contents of which are incorporated herein by reference.
A common type of capacitor implemented as CEXT in the system 10 illustrated in FIG. 1; i.e., one that is available in xe2x80x9cchipxe2x80x9d form with desirable values and voltage ratings, uses a dielectric of the type known in the art as xe2x80x9cX7R.xe2x80x9d While such capacitors provide desirable parametric behavior at relatively low cost, however, they have the undesirable characteristic of a significant fall-off in capacitance at high temperatures. Referring to FIG. 2, for example, a waveform 40 of % capacitance variation over temperature is illustrated for a known and commonly used X7R capacitor having a room temperature (e.g., 25 degrees C) value of 0.22 microfarads. Waveform 40 exhibits a slightly rounded characteristic from approximately xe2x88x9240 degrees C to approximately 60 degrees C, and exhibits a steep roll-off in capacitance starting at approximately 125 degrees C. An over-dwell protection circuit of the type described hereinabove that utilizes a capacitor of the type illustrated in FIG. 2 would accordingly exhibit an increasingly significant reduction in the over-dwell protection time as temperature increases beyond 125 degrees C.
What is therefore needed is a capacitor charging circuit operable to charge capacitor CEXT with a current that compensates for undesirable temperature characteristics of CEXT to thereby minimize timing errors at any given temperature.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention a temperature dependent current generating circuit comprises a first circuit producing a first voltage that is substantially constant over a range of temperatures, a second circuit producing a second voltage as an increasing function of temperature over the range of temperatures, a third current producing a charging current, and a comparator circuit responsive to the first and second voltages to draw a compensation current away from the charging current when the second voltage increases with temperature above the first voltage, wherein the compensation current increases with increasing temperature over the range of temperatures.
In accordance with another aspect of the present invention, a temperature dependent current generating circuit comprises a first circuit producing a compensation current as a function of temperature, and a second circuit producing a charging current, wherein the charging current is a function only of a base charging current below a first temperature and otherwise a function of the base charging current and the compensation current.
One object of the present invention is to provide a temperature dependent current generating circuit.
Another object of the present invention is to provide such a circuit that is useful for charging a capacitor forming part of an automotive ignition system.
Yet another object of the present invention is to provide such a circuit operable to produce a temperature dependent current that compensates for temperature dependent behavior of the capacitor.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiment.